Nickel-base cast alloys containing such elements as chromium, tungsten, cobalt, molybdenum, iron, titanium, silicon, manganese and carbon have generally heat resistance, wear resistance and corrosion resistance superior to those of high-alloy steels, and are therefore popularly in practical use as materials for structural members serving under serious conditions.
In the above mentioned conventional nickel-base cast alloys, however, crystal grains of nickel forming the base grow into coarse dendrites at the time of casting, solidification and cooling thereof. It is also inevitable at the same time that chromium, tungsten and other constituent elements react with carbon, another constituent element, to form carbides which also grow into coarse grains. In addition, as compared with high-alloy steels, the conventional nickel-base cast alloys are far inferior in plastic formability in hot and cold. It is therefore very difficult to refine the above-mentioned base crystal grains and carbide grains thus becoming coarse by forging the alloys.
Because of the aforementioned coarse base crystal grains and carbide grains, in putting the conventional nickel-base cast alloys to practical use, local stress concentration occurring in said coarsening portions often causes breakout of the cast alloys, and moreover, segregation of the constituent elements causes such problems as a decrease in the corrosion resistance of teh cast alloys. It is also very difficult to machine the conventional nickel-base cast alloys into structural members of desired shape and dimensions at a high accuracy because of the very low machinability of such alloys.
With a view to solving the above-mentioned problems inherent to the conventional nickel-base cast alloys, there is proposed the manufacture of nickel-base sintered alloys having substantially the same chemical composition as the nickel-base cast alloys by the powder metallurgy process. The above-mentioned sintered alloys are industrially manufactured in general by the powder metallurgy process, which comprises preparing an alloy powder serving as material powder from a molten alloy containing necessary constituents by the water-atomizing process or the gas-atomizing process; forming a green compact of desired shape and dimensions by pressing said alloy powder after thoroughly pulverizing and mixing said alloy powder; and sintering thus formed green compact with or without a pressure in a reducing, neutral or vacuum atmosphere.
However, the nickel-base sintered alloys manufactured by the above-mentioned conventional powder metallurgy process still have the following drawbacks:
(1) Since the alloy powder serving as the material powder is usually prepared by the water-atomizing or gas-atomizing process, said alloy powder has not only a spherical shape but also a relatively large particle size, this leading to a low compression-formability. What is worse, in sintering, particle surfaces of said alloy powder are not mutually diffused even by heating said alloy powder to a temperature closest to the melting point thereof in a reducing, neutral or vacuum atmosphere. It is therefore difficult to impart to thus manufactured sintered alloy such properties required as a high sintered density and excellent heat resistance, wear resistance and corrosion resistance. PA1 (2) By raising the sintering temperature to a temperature immediately below the melting point of said alloy powder in sintering, it is possible to cause mutual diffusion of particle surfaces of said alloy powder. If the sintering temperature is a little higher than this level, however, said alloy powder is melted down. Thus, the too tight range of sintering temperatures for said alloy powder makes it difficult to apply an effective control over the sintering temperature and hence to go into mass production of a sintered alloy. PA1 (3) As described above, said alloy powder has not only a spherical shape but also a relatively large particle size. Therefore, even if a sintered alloy is manufactured, out of economic considerations, at a sintering temperature within the tight range as mentioned above, it is difficult to impart a sintered density of at least 95% of the theoretical value to the manufactured sintered alloy. In a sintered alloy having sintered density less than 95% of the theoretical value, even with base crystal grains and percipitated carbide grains having a fine and uniform grain size, the toughness decreases under the effect of remaining pores in the sintered alloy, and the heat resistance, impact resistance, the fatigue strength and corrosion resistance, which are properties necessary in actual service, are also decreased.